A multiplexer is a multi-port frequency dependent device that may be used as a separator or combiner of signals falling in different frequency bands. The most common type of multiplexer is a three port device known as a diplexer.
In a multi-band telecommunications device, such as a mobile telephone handset, the respective transmit (TX) and receive (RX) circuitry for each operational frequency band usually share a common antenna. A diplexer circuit is typically used to electrically couple the respective transmit (TX) and receive (RX) circuitry to the antenna. The diplexer ensures that signals in a given frequency band are not directed to the circuitry for a different frequency band.
FIG. 1 shows a simplified block diagram of part of a conventional dual-band telecommunications device such as may be found in a mobile telephone handset. It is assumed that the device is intended for operation in two frequency bands A and B. Hence, the device includes respective TX/RX circuitry 2, 4 for bands A and B respectively, an antenna 6 and a conventional diplexer 8. The diplexer 8 comprises three ports P1, P2 and P3. P1 is connected to the antenna 6, P2 is connected to the TX/RX circuitry of band A and P3 is connected to the TX/RX circuitry of band B. The diplexer 8 comprises two circuit branches, a first between P1 and P2, and a second between P1 and P3. The first branch includes a filter (Filter A) for passing signals in frequency band A. The second branch includes a filter (Filter B) for passing signals in frequency band B. Each branch may also include one or more impedance matching components (not individually shown in FIG. 1) to provide in-band impedance matching.
The arrangement is such that, when a signal in frequency band A is received by antenna 6, Filter A allows the received signal to pass from P1 to P2 and thereby on to the band A TX/RX circuitry 2, while Filter B prevents the received signal from passing from P1 to P3. Similarly, when a signal in frequency band B is received by antenna 6, Filter B allows the received signal to pass from P1 to P3 and thereby on to the band B TX/RX circuitry 4, while Filter A prevents the received signal from passing from P1 to P2.
In the following description, the term “in-band” is intended to refer to the components or characteristics of a circuit or system that relate to the selected or active operational frequency band. The term “out-of-band” is intended to refer to the components or characteristics of a circuit or system that relate to an operational frequency band other than the selected or active operational frequency band. For example, in FIG. 1, when a signal is received in operational frequency band A, the diplexer branch between P1 and P2 may be said to be the in-band branch, whereas the branch between P1 and P3 may be said to be the out-of-band branch.
Ideally, the out-of-band branch of the diplexer 8 should present an open-circuit to the antenna port P1. In practice, however, the out-of-band branch often presents, to the antenna port P1, a finite capacitive or inductive impedance with a high reflection coefficient. The high reflection coefficient enables the out-of-band branch to perform the basic function of preventing the received signal from travelling along the out-of-band branch. However, the finite impedance is commonly relatively low and this is detrimental to the performance of the diplexer and to the device of which it is part (as measured for example, by voltage standing wave ration (VSWR) and insertion loss (IL)).
It would be desirable, therefore, to provide a diplexer, or multiplexer, with improved out-of-band impedance matching. In many applications, including mobile telecommunications, it is important to keep size to a minimum and it would therefore be desirable to provide improved out-of-band matching without adding significantly to the size of the diplexer circuit. It would also be desirable to provide a diplexer, or multiplexer, which provides D.C. decoupling and protection from Electrostatic Discharge (ESD).